Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s...
Transcript of Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s...
![Page 1: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/1.jpg)
1
Chapter 10 Differential Amplifiers
10.1 General Considerations
10.2 Bipolar Differential Pair
10.3 MOS Differential Pair
10.4 Cascode Differential Amplifiers
10.5 Common-Mode Rejection
10.6 Differential Pair with Active Load
![Page 2: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/2.jpg)
CH 10 Differential Amplifiers 2
Audio Amplifier Example
An audio amplifier is constructed as above that takes a rectified AC voltage as its supply and amplifies an audio signal from a microphone.
![Page 3: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/3.jpg)
CH 10 Differential Amplifiers 3
“Humming” Noise in Audio Amplifier Example
However, VCC contains a ripple from rectification that leaks to the output and is perceived as a “humming” noise by the user.
![Page 4: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/4.jpg)
CH 10 Differential Amplifiers 4
Supply Ripple Rejection
Since both node X and Y contain the same ripple, their difference will be free of ripple.
invYX
rY
rinvX
vAvv
vv
vvAv
![Page 5: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/5.jpg)
CH 10 Differential Amplifiers 5
Ripple-Free Differential Output
Since the signal is taken as a difference between two nodes, an amplifier that senses differential signals is needed.
![Page 6: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/6.jpg)
CH 10 Differential Amplifiers 6
Common Inputs to Differential Amplifier
Signals cannot be applied in phase to the inputs of a differential amplifier, since the outputs will also be in phase, producing zero differential output.
0
YX
rinvY
rinvX
vv
vvAv
vvAv
![Page 7: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/7.jpg)
CH 10 Differential Amplifiers 7
Differential Inputs to Differential Amplifier
When the inputs are applied differentially, the outputs are 180° out of phase; enhancing each other when sensed differentially.
invYX
rinvY
rinvX
vAvv
vvAv
vvAv
2
![Page 8: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/8.jpg)
CH 10 Differential Amplifiers 8
Differential Signals
A pair of differential signals can be generated, among other ways, by a transformer.
Differential signals have the property that they share the same average value (DC) to ground and AC values are equal in magnitude but opposite in phase.
![Page 9: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/9.jpg)
CH 10 Differential Amplifiers 9
Single-ended vs. Differential Signals
![Page 10: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/10.jpg)
CH 10 Differential Amplifiers 10
Example 10.3
Determine the common-mode level at the output of the circuit shown in Fig. 10.3(b).
In the absence of signals,
X Y CC C CV V V R I
where 1 2C C CR R R
CI 1Q 2Qdenotes the bias current of and
Thus, CM CC C CV V R I
Interestingly, the ripple affects CMV
but not the differential output.
and
![Page 11: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/11.jpg)
CH 10 Differential Amplifiers 11
Differential Pair
With the addition of a tail current, the circuits above operate as an elegant, yet robust differential pair.
![Page 12: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/12.jpg)
CH 10 Differential Amplifiers 12
Common-Mode Response
2
221
21
EECCCYX
EECC
BEBE
IRVVV
III
VV
2EE
CC C CM
IV R V
To avoid saturation, the collector voltages must not fall below the base voltages:
![Page 13: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/13.jpg)
CH 10 Differential Amplifiers 13
Example 10.4
A bipolar differential pair employs a load resistance of 1 kΩand a tail current of 1 mA. How close to VCC can VCM be chosen?
2EE
CC CM C
IV V R
0 5V
CMVCCVThat is, must remain below
by at least 0.5 V.
![Page 14: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/14.jpg)
CH 10 Differential Amplifiers 14
Common-Mode Rejection
Due to the fixed tail current source, the input common-mode value can vary without changing the output common-mode value.
![Page 15: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/15.jpg)
CH 10 Differential Amplifiers 15
Differential Response I
CCY
EECCCX
C
EEC
VV
IRVV
I
II
02
1
![Page 16: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/16.jpg)
CH 10 Differential Amplifiers 16
Differential Response II
CCX
EECCCY
C
EEC
VV
IRVV
I
II
01
2
![Page 17: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/17.jpg)
CH 10 Differential Amplifiers 17
Differential Pair Characteristics
None-zero differential input produces variations in output currents and voltages, whereas common-mode input produces no variations.
![Page 18: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/18.jpg)
CH 10 Differential Amplifiers 18
Example 10.5
A bipolar differential pair employs a tail current of 0.5 mA and a collector resistance of 1 kΩ. What is the maximum allowable base voltage if the differential input is large enough to completely steer the tail current? Assume VCC=2.5V.
Because is completely steered,
- 2 at one collector.
To avoid saturation, 2 .
EE
CC C EE
B
I
V R I V
V V
![Page 19: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/19.jpg)
CH 10 Differential Amplifiers 19
Small-Signal Analysis
Since the input to Q1 and Q2 rises and falls by the same amount, and their emitters are tied together, the rise in IC1
has the same magnitude as the fall in IC2.
II
I
II
I
EEC
EEC
2
2
2
1
![Page 20: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/20.jpg)
CH 10 Differential Amplifiers 20
Virtual Ground
For small changes at inputs, the gm’s are the same, and the respective increase and decrease of IC1 and IC2 are the same, node P must stay constant to accommodate these changes. Therefore, node P can be viewed as AC ground.
VgI
VgI
V
mC
mC
P
2
1
01 ( )C m PI g V V
2 ( )C m PI g V V
( ) ( )m P m Pg V V g V V
2 1 0C CI I
![Page 21: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/21.jpg)
CH 10 Differential Amplifiers 21
Small-Signal Differential Gain
Since the output changes by -2gmVRC and input by 2V, the small signal gain is –gmRC, similar to that of the CE stage. However, to obtain same gain as the CE stage, power dissipation is doubled.
CmCm
v RgV
VRgA
2
2
X m CV g VR
Y m CV g VR
2X Y m CV V g VR
![Page 22: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/22.jpg)
CH 10 Differential Amplifiers 22
Example 10.6
Design a bipolar differential pair for a gain of 10 and a power budget of 1mW with a supply voltage of 2V.
2 V
1 mW0.5 mA
2 V
/ 2 0.25 mA 1
26 mV 104
1040
CC
EE
C EEm
T T
vC
m
V
I
I Ig
V V
AR
g
![Page 23: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/23.jpg)
CH 10 Differential Amplifiers 23
Example 10.7
Compare the power dissipation of a bipolar differential pair with that of a CE stage if both circuits are designed for equal voltage gains, collector resistances, and supply voltages.
Differential pair CE stage
diff 1 2V m CA g R V CE m CA g R
1 2m C m Cg R g R
2CEE
T T
II
V V
2EE CI I
, 2D diff C EE C CP V I V I ,D CE C CP V I
![Page 24: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/24.jpg)
CH 10 Differential Amplifiers 24
Large Signal Analysis
1 2 1 2
1 2
1 2
1 2
1 22 2
21 2
1 2
11 2
ln ln
and
exp
1 exp
exp
1 exp
in in BE BE
C CT T
S S
C C EE
in inC C EE
T
EEC
in in
T
in inEE
TC
in in
T
V V V V
I IV V
I I
I I I
V VI I I
V
II
V V
V
V VI
VI
V V
V
![Page 25: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/25.jpg)
CH 10 Differential Amplifiers 25
Example 10.8
Determine the differential input voltage that steers 98% of the tail current to one transistor.
1 0 02C EEI I
1 2exp in inEE
T
V VI
V
1 2 3 91in in TV V V
We often say a differential input of 4 TVis sufficient to turn one side of the bipolarpair nearly off.
![Page 26: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/26.jpg)
CH 10 Differential Amplifiers 26
Input/Output Characteristics
1 1
1 2
1 2
2 2
1 2
exp
1 exp
1 exp
out CC C C
in inEE
TCC C
in in
T
out CC C C
EECC C
in in
T
V V R I
V VI
VV R
V V
V
V V R I
IV R
V V
V
1 2
1 21 2
1 2
1 exp
1 exp
tanh2
in in
Tout out C EE
in in
T
in inC EE
T
V V
VV V R I
V V
V
V VR I
V
![Page 27: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/27.jpg)
CH 10 Differential Amplifiers 27
Example 10.9
Sketch the output waveforms of the bipolar differential pair in Fig. 10.14(a) in response to the sinusoidal inputs shown in Figs. 10.14(b) and (c). Assume Q1 and Q2 remain in the forward active region.
Figure10.14 (a)
![Page 28: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/28.jpg)
CH 10 Differential Amplifiers 28
Example 10.9 (cont’d)
The left column operates in linear region (small-signal), whereas the right column operates in nonlinear region (large-signal).
![Page 29: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/29.jpg)
CH 10 Differential Amplifiers 29
Small-Signal Model
1 1 2 2in P inv v v v v
1 21 1 2 2
1 2
0m m
v vg v g v
r r
1 2r r 1 2m mg gWith and 1 2v v
Since 1 2in inv v 1 12 2inv v
1 1
0
P inv v v
, .
![Page 30: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/30.jpg)
CH 10 Differential Amplifiers 30
Half Circuits
Since VP is grounded, we can treat the differential pair as two CE “half circuits”, with its gain equal to one half circuit’s single-ended gain.
Cm
inin
outout Rgvv
vv
21
21
![Page 31: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/31.jpg)
CH 10 Differential Amplifiers 31
Example 10.10
Compute the differential gain of the circuit shown in Fig. 10.16(a), where ideal current sources are used as loads to maximize the gain.
Figure10.16 (a) Om
inin
outout rgvv
vv
21
21
![Page 32: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/32.jpg)
CH 10 Differential Amplifiers 32
Example 10.11
Figure 10.17(a) illustrates an implementation of the topology shown in Fig. 10.16(a). Calculate the differential voltage gain.
Figure10.17 (a)
1 2
1 2
out outm ON OP
in in
v vg r r
v v
![Page 33: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/33.jpg)
CH 10 Differential Amplifiers 33
Extension of Virtual Ground
It can be shown that if R1 = R2, and points A and B go up and down by the same amount respectively, VX does not move. This property holds for any other node that appears on the axis of symmetry.
0XV
![Page 34: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/34.jpg)
CH 10 Differential Amplifiers 34
Half Circuit Example I
1311 |||| RrrgA OOmv
![Page 35: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/35.jpg)
CH 10 Differential Amplifiers 35
Half Circuit Example II
1311 |||| RrrgA OOmv
![Page 36: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/36.jpg)
CH 10 Differential Amplifiers 36
Half Circuit Example III
m
E
Cv
gR
RA
1
![Page 37: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/37.jpg)
CH 10 Differential Amplifiers 37
Half Circuit Example IV
m
E
Cv
g
R
RA
1
2
![Page 38: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/38.jpg)
CH 10 Differential Amplifiers 38
I/O Impedances
1 2
1 2
1 2
1 2
X
X
X
v vi
r r
v v v
r i
vX
iX
vX
iX iX
In a similar manner,
2out CR R
12Xin
X
vR r
i
![Page 39: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/39.jpg)
CH 10 Differential Amplifiers 39
MOS Differential Pair’s Common-Mode Response
Similar to its bipolar counterpart, MOS differential pair produces zero differential output as VCM changes.
1 22
2
SSD D
SSX Y DD D
II I
IV V V R
![Page 40: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/40.jpg)
CH 10 Differential Amplifiers 40
Equilibrium Overdrive Voltage
The equilibrium overdrive voltage is defined as the overdrive voltage seen by M1 and M2 when both of them carry a current of ISS/2.
2
2
1
2
SSD
n ox GS TH
SSGS TH equil
n ox
II
WC V V
L
IV V
WC
L
![Page 41: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/41.jpg)
CH 10 Differential Amplifiers 41
Minimum Common-mode Output Voltage
In order to maintain M1 and M2 in saturation, the common-mode output voltage cannot fall below the value above.
This value usually limits voltage gain.
THCMSS
DDD VVI
RV 2
![Page 42: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/42.jpg)
CH 10 Differential Amplifiers 42
Example 10.15
A MOS differential pair is driven with an input CM level of 1.6V. If ISS=0.5mA, VTH=0.5 V, and VDD=1.8 V, what is the maximum allowable load resistance?
2
2
2.8 k
SSDD D CM TH
DD CM THD
SS
IV R V V
V V VR
I
![Page 43: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/43.jpg)
CH 10 Differential Amplifiers 43
Differential Response
![Page 44: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/44.jpg)
CH 10 Differential Amplifiers 44
Small-Signal Response
Similar to its bipolar counterpart, the MOS differential pair exhibits the same virtual ground node and small signal gain.
1
2
0
,
2
P
D m
D m
X Y m D
v m D
V
I g V
I g V
V V g R V
A g R
![Page 45: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/45.jpg)
CH 10 Differential Amplifiers 45
Power and Gain Tradeoff
In order to obtain the same gain as a CS stage, a MOS differential pair must dissipate twice the amount of current. This power and gain tradeoff is also echoed in its bipolar counterpart.
![Page 46: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/46.jpg)
CH 10 Differential Amplifiers 46
Example 10.16
Design an NMOS differential pair for a voltage gain of 5 and a power budget of 2 mW subject to the condition that the stage following the differential pair requires an output CM level of at least 1.6V. Assume μnCox=100 μA/V2, λ=0, and VDD= 1.8 V.
,
2 mW1 11 mA
1.8 V
1.6 V2
360
For 360 ,
52
360 2
1738
SS
SSCM out DD D
D
D
v SSm n ox
D
I
IV V R
R
R
A IWg C
R L
W
L
Textbook Error!
![Page 47: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/47.jpg)
CH 10 Differential Amplifiers 47
Example 10.17
What is the maximum allowable input CM level in the previous example if VTH=0.4 V?
1 2
,
,
,
To guarantee that M and M
operate in saturation,
2
.
,
2 V
SSCM in DD D TH
CM out TH
CM in
IV V R V
V V
Thus
V
![Page 48: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/48.jpg)
CH 10 Differential Amplifiers 48
Example 10.18
The common-source stage and the differential pair shown in Fig. 10.28 incorporate equal load resistors. If the two circuits are designed for the same voltage gain and the same supply voltage, discuss the choice of (a) transistor dimensions for a given power budget, (b) power dissipation for given transistor dimensions.
Figure10.28
3
1 2 3
31 2
1 2 3
1
(a) for same power budget,
2 2
1 1
2 2
2
(b) for same transistor dimensions,
2
2
D SS D D
m n ox D
SS D
diff CS
I I I I
WW W
L L L
Wg C I
L
I I
P P
![Page 49: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/49.jpg)
CH 10 Differential Amplifiers 49
MOS Differential Pair’s Large-Signal Response
1 1 2 2.in GS in GSV V V V
1 2D D SSI I I
2(1 2) ( )( ) .D n ox GS THI C W L V V
2 DGS TH
n ox
IV V
WC
L
1 2 1 2in in GS GSV V V V
1 2
2( )D D
n ox
I IW
CL
Goal is to obtain ID1 - ID2
![Page 50: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/50.jpg)
CH 10 Differential Amplifiers 50
MOS Differential Pair’s Large-Signal Response (cont’d)
21 2 1 2 1 2
1 2
2( ) ( 2 )
2( 2 )
in in D D D D
n ox
SS D D
n ox
V V I I I IW
CL
I I IW
CL
21 2 1 24 2 ( )D D SS n ox in in
WI I I C V V
L
2
21 2 1 216 2 ( )D D SS n ox in in
WI I I C V V
L
2
21 1 1 216 ( ) 2 ( )D SS D SS n ox in in
WI I I I C V V
L
![Page 51: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/51.jpg)
CH 10 Differential Amplifiers 51
MOS Differential Pair’s Large-Signal Response (cont’d)
2
21121
4
2
12 inin
oxn
SSinoxnDD VV
L
WC
IVV
L
WCII
in
2
2 21 1 1 216 16 2 ( ) 0D SS D SS n ox in in
WI I I I C V V
L
2
2 21 1 2
14 ( ) 2
2 4SS
D SS n ox in in SS
I WI I C V V I
L
21 21 24 ( )
2 4SS in in
n ox SS n ox in in
I V V W WC I C V V
L L
22 12 2 14 ( )
2 4
( the symmetry of the circuit)
SS in inD n ox SS n ox in in
I V V W WI C I C V V
L L
![Page 52: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/52.jpg)
CH 10 Differential Amplifiers 52
Maximum Differential Input Voltage
There exists a finite differential input voltage that completely steers the tail current from one transistor to the other. This value is known as the maximum differential input voltage.
1 2 max
22SS
in in GS TH equil
n ox
IV V V V
WC
L
~0 ISS
Edge of Conduction
VTH VGS2
+ +
- -
1GS THV V
2
2 SSGS TH
n ox
IV V
WC
L
![Page 53: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/53.jpg)
CH 10 Differential Amplifiers 53
Contrast Between MOS and Bipolar Differential Pairs
In a MOS differential pair, there exists a finite differential input voltage to completely switch the current from one transistor to the other, whereas, in a bipolar pair that voltage is infinite.
MOS Bipolar
![Page 54: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/54.jpg)
CH 10 Differential Amplifiers 54
The effects of Doubling the Tail Current
Since ISS is doubled and W/L is unchanged, the equilibrium overdrive voltage for each transistor must increase by to accommodate this change, thus Vin,max increases by as well. Moreover, since ISS is doubled, the differential output swing will double.
22
![Page 55: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/55.jpg)
CH 10 Differential Amplifiers 55
The effects of Doubling W/L
Since W/L is doubled and the tail current remains unchanged, the equilibrium overdrive voltage will be lowered by to accommodate this change, thus Vin,max will be lowered by as well. Moreover, the differential output swing will remain unchanged since neither ISS nor RD has changed
2
![Page 56: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/56.jpg)
CH 10 Differential Amplifiers 56
Example 10.20
Design an NMOS differential pair for a power budget of 3 mW and ∆Vin,max=500 mV. Assume μnCox=100 μA/V2 and VDD=1.8 V.
1 2 max
2,max
3 mW1.67 mA
1.8 V
2From
2133.6
is determined by the required voltage gain.
SS
SSin in
n ox
SS
n ox in
D
I
IV V
WC
L
IW
L C V
R
![Page 57: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/57.jpg)
CH 10 Differential Amplifiers 57
Small-Signal Analysis of MOS Differential Pair
When the input differential signal is small compared to [4ISS/nCox(W/L)]1/2, the output differential current is linearly proportional to it, and small-signal model can be applied.
2
1 2 1 2 1 2
1 2
1 2
41
2
41
2
SSD D n ox in in in in
n ox
SSn ox in in
n ox
n ox SS in in
IWI I C V V V V
WL CL
IWC V V
WL CL
WC I V V
L
![Page 58: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/58.jpg)
CH 10 Differential Amplifiers 58
Virtual Ground and Half Circuit
Applying the same analysis as the bipolar case, we will arrive at the same conclusion that node P will not move for small input signals and the concept of half circuit can be used to calculate the gain.
0P
v m D
V
A g R
![Page 59: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/59.jpg)
CH 10 Differential Amplifiers 59
MOS Differential Pair Half Circuit Example I
13
3
1 ||||1
0
OO
m
mv rrg
gA
![Page 60: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/60.jpg)
CH 10 Differential Amplifiers 60
MOS Differential Pair Half Circuit Example II
3
1
0
m
mv
g
gA
![Page 61: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/61.jpg)
CH 10 Differential Amplifiers 61
MOS Differential Pair Half Circuit Example III
mSS
DDv
gR
RA
12
2
0
![Page 62: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/62.jpg)
CH 10 Differential Amplifiers 62
Bipolar Cascode Differential Pair
1 3 1 3 3 1 3|| ||v m m O O OA g g r r r r r
![Page 63: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/63.jpg)
CH 10 Differential Amplifiers 63
Output Impedance of CE Stage with Degeneration
|| ||
1 ( || ) ||
( 1)( || )
1 ( || )
X X m O
X m X E O X E
out m E O E
O m O E
O m E
v i g v r v
i g i R r r i R r
R g R r r R r
r g r R r
r g R r
![Page 64: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/64.jpg)
CH 10 Differential Amplifiers 64
Output Impedance of CS Stage with Degeneration
1
1
1
X m X m X S X m X S
O X m X S X S X
out O m S S
m O S O
m O S O
i g v i g i R i g i R
r i g i R i R v
R r g R R
g r R r
g r R r
![Page 65: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/65.jpg)
CH 10 Differential Amplifiers 65
Bipolar Telescopic Cascode
)||(|||| 575531331 rrrgrrrggA OOmOOmmv
![Page 66: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/66.jpg)
CH 10 Differential Amplifiers 66
Example: Bipolar Telescopic Parasitic Resistance
1 15 5 7 5 7 5
1 3 3 1 3
1 || || || ||2 2
( || ) ||
op O m O O
v m m O O op
R RR r g r r r r
A g g r r r R
![Page 67: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/67.jpg)
CH 10 Differential Amplifiers 67
MOS Cascode Differential Pair
1331 OmOmv rgrgA
![Page 68: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/68.jpg)
CH 10 Differential Amplifiers 68
MOS Telescopic Cascode
)(|| 7551331 OOmOOmmv rrgrrggA
![Page 69: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/69.jpg)
CH 10 Differential Amplifiers 69
Example: MOS Telescopic Parasitic Resistance
5 5 1 7 5 1
1 3 3 1
1 || ||
( || )
op m O O O
v m op O m O
R g r R r r R
A g R r g r
![Page 70: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/70.jpg)
CH 10 Differential Amplifiers 70
Effect of Finite Tail Impedance
If the tail current source is not ideal, then when a input CM voltage is applied, the currents in Q1 and Q2 and hence output CM voltage will change.
mEE
C
CMin
CMout
gR
R
V
V
2/1
2/
,
,
![Page 71: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/71.jpg)
CH 10 Differential Amplifiers 71
Input CM Noise with Ideal Tail Current
![Page 72: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/72.jpg)
CH 10 Differential Amplifiers 72
Input CM Noise with Non-ideal Tail Current
![Page 73: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/73.jpg)
CH 10 Differential Amplifiers 73
Comparison
As it can be seen, the differential output voltages for both cases are the same. So for small input CM noise, the differential pair is not affected.
![Page 74: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/74.jpg)
CH 10 Differential Amplifiers 74
CM to DM Conversion, ACM-DM
If finite tail impedance and asymmetry are both present, then the differential output signal will contain a portion of input common-mode signal.
1 2
2
1 2 ,
12
1/ 2
1/ 2 2
CM GS D SS
DD SS GS
m m
CMD
SS
m
out out out
D D D D D
D D
CMD
m SS
out D D
CM m SS SS
V V I R
II R V
g g
VI
Rg
V V V
I R I R R
I R
VR
g R
V R R
V g R R
2
1 1
2
2 2
1 2 1 2 1 2
1
2
1
2
& ,
D n ox GS TH
D n ox GS TH
D D D D GS GS
WI C V V
L
WI C V V
L
I I I I V V
Textbook Error!
![Page 75: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/75.jpg)
CH 10 Differential Amplifiers 75
Example: ACM-DM
313313
1
||)]||(1[21
rRrrRgg
R
A
Om
m
C
DMCM
![Page 76: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/76.jpg)
CH 10 Differential Amplifiers 76
CMRR
CMRR defines the ratio of wanted amplified differential input signal to unwanted converted input common-mode noise that appears at the output.
DMCM
DM
A
ACMRR
![Page 77: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/77.jpg)
Example 10.28
Calculate the CMRR of the circuit in Fig. 10.46.
CH 10 Differential Amplifiers 77
Figure10.46
1
13 1 3 3 1 3
1
1 2 [1 ( || )] ||
m CDM
CM DM CM DM
m Cm O
C m
g RACMRR
A A
g Rg R r r R r
R g
![Page 78: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/78.jpg)
CH 10 Differential Amplifiers 78
Differential to Single-ended Conversion
Many circuits require a differential to single-ended conversion, however, the above topology is not very good.
![Page 79: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/79.jpg)
CH 10 Differential Amplifiers 79
Supply Noise Corruption
The most critical drawback of this topology is supply noise corruption, since no common-mode cancellation mechanism exists. Also, we lose half of the signal.
![Page 80: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/80.jpg)
CH 10 Differential Amplifiers 80
Better Alternative
This circuit topology performs differential to single-ended conversion with no loss of gain.
![Page 81: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/81.jpg)
CH 10 Differential Amplifiers 81
Active Load
With current mirror used as the load, the signal current produced by the Q1 can be replicated onto Q4.
This type of load is different from the conventional “static load” and is known as an “active load”.
![Page 82: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/82.jpg)
CH 10 Differential Amplifiers 82
Differential Pair with Active Load
The input differential pair decreases the current drawn from RL by I and the active load pushes an extra I into RL by current mirror action; these effects enhance each other.
![Page 83: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/83.jpg)
CH 10 Differential Amplifiers 83
Active Load vs. Static Load
The load on the left responds to the input signal and enhances the single-ended output, whereas the load on the right does not.
![Page 84: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/84.jpg)
CH 10 Differential Amplifiers 84
MOS Differential Pair with Active Load
Similar to its bipolar counterpart, MOS differential pair can also use active load to enhance its single-ended output.
![Page 85: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/85.jpg)
CH 10 Differential Amplifiers 85
Asymmetric Differential Pair
Because of the vastly different resistance magnitude at the drains of M1 and M2, the voltage swings at these two nodes are different and therefore node P cannot be viewed as a virtual ground when Vin2=-Vin1.
![Page 86: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/86.jpg)
Quantitative Analysis - Approach 1
CH 10 Differential Amplifiers 86
Av
N N
OPrOP
mP
1rg
1M 2M
3M 4M
ONrONr
mP Ag vA
YiXi
outv
1
1
1
&
1
X Y A X mP OP
out outY mP A mP X mP OP X
OP OP
outX
OP mP mP OP
i i v i g r
v vi g v g i g r i
r r
vi
r g g r
![Page 87: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/87.jpg)
Quantitative Analysis - Approach 1 – cont’d
CH 10 Differential Amplifiers 87
1 2
1 2
1 2 1 2
1
1 1
0
2 0
&
Substituting for and ,
21 1
A X mN ON Y mN ON out
A X ON mN ON in in out
in in X Y
A X
out outmP OP ON
OP mP mP OP OP mP mP OP
v i g v r i g v r v
v i r g r v v v
v v v v i i
v i
v vg r r
r g g r r g g r
1 2
1
1 2
1
2 2
out mN ON in in
OP mP mP OPoutmN ON
in in ON OP
mN ON OP
v g r v v
r g g rvg r
v v r r
g r r
![Page 88: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/88.jpg)
CH 10 Differential Amplifiers 88
Quantitative Analysis - Approach 2
1 2
1 2 1 1 1 2 2 2
( )
2
&
Thev mN oN in in
Thev oN
X m O X m O X
v g r v v
R r
v v i g v r i g v r v
vX iX
v1 v2
Textbook Error!
Textbook Error!
+ -
iX
![Page 89: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/89.jpg)
CH 10 Differential Amplifiers 89
Quantitative Analysis - Approach 2 – cont’d
3
3
3
3
4
43
3
1
1
01
O
mA out Thev
O Thev
m
out out Thevm A
OO Thev
m
rg
v v v
r Rg
v v vg v
r r Rg
3
34
43 3
3 3
3 3 4
4 3
1 2
1 2
1
10
1 1
2 10 ( and )
1 1
O
m outm out Thev
OO Thev O Thev
m m
outout Thev O Thev m m
Thev O m
mN ON in inout
ON OP ON
outmN ON OP
in in
rg v
g v vrr R r R
g g
vv v r R g g
R r g
g r v vv
r r r
vg r r
v v
+ -
![Page 90: Chapter 10 Differential AmplifiersCH 10 Differential Amplifiers 39 MOS Differential Pair’s Common-Mode Response Similar to its bipolar counterpart, MOS differential pair produces](https://reader035.fdocuments.in/reader035/viewer/2022062403/610c052682d20009341c19da/html5/thumbnails/90.jpg)
CH 10 Differential Amplifiers 90
I
A
Example 10.29
4 3
4 3
4
4 3
4 3
4 3
1
1
2
outA m A O
O m
outm A
O m
outA A
O m
outA
O m
vv g v r
r g
vg v
r g
vv v
r g
vv
r g
Prove that the voltage swing at node A is much less than that at the output.
Am
O
out vgr
vI 4
4